The balancing device including static dynamic balancing weights, which are housed in and/or supported by this external end portion of the main blade section, for a good effectiveness of these adjustment weights, which can be placed and held on at least one weight support mounted on the blade.
These weights are used to obtain the static and dynamic adjustment or balancing of a blade. The static adjustment enables the static moment of each blade to be adjusted relative to a standard, this static moment being the product of the mass of the blade and the distance separating, along the blade span, the centre of gravity of the blade from the rotor rotational axis. Respect of this static moment criterion avoids fatigue of the rotor mast, rotating the hub and the rotor blades, caused by an excessive unbalance resulting from too different static moments from one blade to another, and contributes to the comfort and increases the life span of the rotor by decreasing rotor vibrations. This static adjustment enables in a global way the mass tolerances of all the components of a blade to be corrected, such as its spar(s), its extrados and intrados coatings, its padding(s), etc. The maximum capacity of the static adjustment is in general of the order of 1 to 2% of the mass of the blade, and the sensitivity of this adjustment is of the order of a few grams for blades of several dozens of kilograms.
The static adjustment operation consists in placing the blade on a beam provided with a balance, and adding to the blade the necessary quantity of balancing weights to increase the mass of the blade so as to obtain the standard static moment. The balancing weights thus added are neutral in relation to the chord centring, i.e. placed on the chord centring axis or equally distributed either side of this axis, which is a longitudinal axis, extending along the blade span, and passing through the centres of gravity of the successive basic profiled sections of the blade. In general, the chord centring axis is located substantially at 25% of the chord from the blade leading edge (i.e. in the front quarter of the chord) and at mid-thickness of the profile, this chord centring axis, or the blade neutral axis with regard to the chord centring, being able to be merged with the blade twisting axis.
The dynamic adjustment or balancing enables the chord moment of each blade to be adjusted relative to a standard blade, this chord moment being the product of the blade mass and a distance, measured along the chord, between the centre of gravity of the blade and, in principle, the theoretical 20% chord axis from the leading edge corresponding substantially to the position of the centre of gravity along this chord. This adjustment enables, in a global way, the mass defects of all the components of the blade to be corrected, as well as the small profile defects of the blade. This dynamic adjustment is generally completed by an action on tabs or small trailing edge flaps, which are bent in order to give a particular angle of incidence. The capacity of the dynamic adjustment is determined by an empirical formula in which occur parameters such as the blade mass, the position of the weight support(s), etc.
The dynamic adjustment operation consists in comparing, on a test bench, the aerodynamic behaviour of the blade to be adjusted relative to a standard blade, and to transfer, along the chord, blade end masses to the front or to the rear of the blade, between positions which are located either side the chord centring axis, until the aerodynamic behaviour of the blade is in practice made identical to that of the standard blade.
A blade end static and dynamic balancing device enables therefore the balancing of blades intended for a rotor to be adjusted in order to make them interchangeable one by one on this rotor.
Balancing devices are already known for which the weights are placed on blade end pins. A light alloy weight support is fixed, for example by gluing, at the blade end between the extrados and intrados coatings of the blade, and three pins are screwed on this support parallel to one another and along the blade span, so as to project longitudinally outwards from the end of the main blade section of the blade, and the static and dynamic weights are mounted on these pins and retained on them by nuts. The static weights can be supported by the central pin, aligned on the chord centring axis, whereas the dynamic weights, which also contribute to the static balancing of the blade, are distributed on the two other pins, symmetrical with the central pin to the front and to the rear of the blade.
The disadvantages of such a device are that it presents a risk of corrosion at the level of the light alloy weight support. Furthermore, the pins are subjected to bending moments during acceleration and braking phases of the rotor (temporary fatigue), and it is necessary to dismantle the blade tip cap, which encompasses the weights and the portions of the pins supporting them, and which is added to the blade tip, on the end of the main blade section of the blade, in order to have access to the weights during adjustments. Because of this arrangement of the weights, the device presents also great vulnerability in the event of shock to the blade tip, and such a device is incompatible with numerous developed shapes of cap or blade tip, such as the parabolic, serrated, dihedral shapes, etc.
Static and dynamic balancing devices are also known for which the weights are housed in cavities made in the external end portion of the blade main section and which emerge in the external end section of this main blade section. In general, the static and dynamic balancing weights are placed in cylindrical cavities delimited in part by stainless steel tapped tubes which are extended by tubular parts in composite material. The whole is made during the moulding of the blade, in the case of blades mainly of composite material, and forms an integral part of the blade structure. The cavities are closed by stainless steel threaded plugs, held by resilient pins, and accessible on the external end section of the main blade section of the blade.
Such a balancing device avoids numerous disadvantages of a pin type device presented above, but always necessitates the dismantling of the blade tip cap in order to have access to the threaded plugs then to the balancing weights. Moreover, such a device is also incompatible with numerous developed forms of cap or blade tip, with parabolic, serrated or dihedral shapes, etc.
Again static and dynamic balancing devices are known for which the weights are placed in cavities which emerge in the extrados surface of the blade. Casings, formed for example from composite material blocks, generally containing glass fibre, are glued during the moulding of the blade between the extrados and intrados coatings of the blade, and the cavities intended to receive the static and dynamic weights are made in the casings either directly by moulding by means of cinblots, or bored after moulding the blade, the extrados coating of which has access openings to these cavities. Stainless steel doors are fixed on the extrados coating of the blade, with the aid of screws or rivets, in order to close the cavities after the static and dynamic weights have been placed in them. These doors are curved and integrated with the blade profile.
Such a device therefore makes it unnecessary to intervene on the blade tip cap for every intervention on the balancing weights. Furthermore, such a device is compatible with all developed shapes of cap or blade tip, added or forming an integrating part of the blade, since the accessibility to the cavities and to the weights is enabled by the easily accessible doors on the blade extrados, in the external end portion of its main blade section, and which do not therefore extend over the cap.
Such a balancing device comprises however disadvantages: the openings made in the extrados coating are significant, and must therefore be accompanied with reinforcement. The weights are directly supported on the doors, and consequently the doors must be sufficiently rigid to retain the weights subjected to accelerations. The stiffness of the doors is not homogeneous with that of the blade, which leads to sealing problems of the cavities. The respect for the blade profile, particularly important at the end of the blade, necessitates the making of doors curved along the blade profile. Finally, the fixing of the doors is stressing, since the weights are supported on the doors, and consequently requires either fixing by double locking screws, for conformity with aeronautical standards, or a non detachable mounting, with rivets for example.